1. Field of the Invention
The present invention relates to a printed circuit board and pad apparatus having a solder deposit, and more particularly, to a printed circuit board and pad apparatus having a solder deposit formed on the pad apparatus by using a mask having a slit having wide ends, a narrow web, and tapered portions connected between the narrow web and one of the wide ends.
2. Description of the Prior Art
A surface mounted device (xe2x80x9cSMDxe2x80x9d), such as a resistor, a capacitor, an inductor, a jumper, etc, includes a plurality of conductive leads mounted on and electronically connected to a plurality of conductive pads formed on a printed circuit board (xe2x80x9cPCBxe2x80x9d). The pads includes two opposite and spaced-apart conductive ends and a nonconductive intermediate portion disposed between the two opposite and spaced-apart conductive ends. The SMD is located between the two opposite and spaced-apart conductive ends and on the nonconductive intermediate portion while each lead of the SMD is seated on each opposite conductive end. The SMD is electrically connected to the pads of the PCB by soldering the leads of the SMD to the pads. Each pad has a solder deposit for electrically and firmly connecting the pad to the lead. The solder deposit has been typically formed on the pads of the PCB using a mask, such as a xe2x80x9cmeshxe2x80x9d or a xe2x80x9cstencil.xe2x80x9d
The SMD has evolved to a variety of semiconductor devices, such as plastic quad flat packages (PQFPs) and quad flat packages (QFPs), and a plurality of expansion connectors. The semiconductor devices and connectors includes a plurality of leads extended from a main body of the semiconductor devices and connectors and having a very small It ad pitch between the adjacent leads pin compared to other types of semiconductor devices and connectors. Due to the small lead pitch, the semiconductors and connectors are undesirably short-circuited since a solder applied to each pair of the pad and the lead easily and electrically connect the lead to the adjacent lead, the pad to the adjacent pad, or the pad to the adjacent lead during a soldering process. Particularly, in the case of semiconductor devices or connectors having a lead pitch less than 0.5 mm (millimeters), the defective soldering of the adjacent leads of the semiconductor devices and connectors or the adjacent pads of the PCB causes a short circuit and leads to an entire defective unit containing the PCB and the semiconductor devices or connectors.
U.S. Pat. No. 4,645,114 entitled Shaped Solder Pad For Surface Mounting Electronic Devices And A Surface Mounting Portion Incorporating Such Shaped Pads to Van Den Brekel et al., U.S. Pat. No. 5,303,112 entitled Printed Circuit Board Having A Commonized Pad Which Different Sized Surface Mounted Devices Can Be Mounted to Adams, Jr. et al., U.S. Pat. No. 4,870,225 entitled Mounting Arrangement Of Chip Type Component Onto Printed Circuit Board to Anao et al., U.S. Pat. No. 5,330,096 entitled Method Of Wave Soldering With Unique Solder Pad Configuration to Belanger, Jr., U.S. Pat. No. 5,406,458 entitled Printed Circuit Board Having Tapered Contact Pads For Surface Mounted Electrical Components to Schutt, U.S. Pat. No. 5,453,581 entitled Pad Arrangement For Surface Mount Components to Liebman et al., and U.S. Pat. No. 5,825,628 entitled Electronic Package With Enhanced Pad Design to Garbelli et al. disclose at least two conductive end portions spaced-apart from each other, a nonconductive portion disposed between the two opposite end portions, and two leads each located on each corresponding conductive end portion. These references, however, do not show particular shape of a pad having a solder deposit assigned to a lead and preventing a short circuit formed between the adjacent leads, the adjacent pad, or the adjacent lead and pad.
Although U.S. Pat. No. 5,428,505 entitled Printed Circuit Board to Sakemi et al., U.S. Pat. No. 5,446,244 entitled Printed Wiring Pattern to Kawanabe et al., U.S. Pat. No. 5,534,667 entitled Printed Circuit Board Having Zig-Zag Contact Arrangement to Miyamoto et al., and U.S. Pat. No. 5,468,920 entitled Printed Circuit Board Having Raised Conductor Pads to August disclose a pad and a lead connected to the pad, these references fail to show a particular shape of a pad preventing a short circuit formed between the adjacent leads or the adjacent pad and uniformly applying a solder to the pad by using a mask having a slit identical to the particular shape of the pad.
U.S. Pat. No. 5,511,306 entitled Masking Of Circuit Board Vias to Reduce Heat-Induced Board And Chip Carrier Package Warp During Wavesolder Process to Denton et al., U.S. Pat. No. 5,866,940 entitled Chip Scale Ball Grid Array For Integrated Circuit Packaging to Schueller, U.S. Pat. No. 5,875,102 entitled Eclipse Via In Pad Structure to Barrow, and U.S. Pat. No. 5,892,290 entitled Highly Reliable And Planar Ball Grid Array Package to Chakravorty et al. disclose a variety of pad formed on a ball grid array or a quad flat pack. Also, these references do not show a pad having a solder deposit preventing a short circuit formed between the adjacent leads or the adjacent pad and uniformly applying a solder to the pad by using a mask having a slit identical to the particular shape of the solder deposit of the pad.
It is an object of the present invention to provide a pad apparatus formed on a printed circuit board able to prevent a surface mounted device having a plurality of narrow leads from being electrically connected between the leads during soldering of each lead of the surface mounted device on a solder deposit of the pad apparatus.
It is another object to provide a pad apparatus formed on a printed circuit board able to connect leads of a surface mounted device to the printed circuit board without causing a short circuit between the adjacent leads, the adjacent pads, or the adjacent lead and pad.
It is yet another object to provide a pad apparatus formed on a printed circuit board able to prevent a solder deposit formed on the pad apparatus from flowing over outside the pad apparatus during mounting of a surface mounted device having a plurality of narrow leads on the printed circuit board.
It is still another object to provide a process of uniformly applying a solder on an entire area of a solder deposit of the pad apparatus through a mask including a slit having the same shape as the solder deposit.
It is a further object to provide a pad apparatus formed on a printed circuit board able to prevent the pad apparatus from blocking a solder to be uniformly formed on an entire area of a solder deposit of the pad apparatus.
It is also an object to provide a pad apparatus formed on a printed circuit board able to stably, firmly, and effectively solder narrow leads of semiconductor devices and expansion connectors to the pad apparatus.
These and other objects may be achieved by providing a printed circuit board (xe2x80x9cPCBxe2x80x9d) having a conductive pad formed on a board of the PCB and used for mounting a lead of an electronic device on the board, and a solder deposit formed on the pad and used for electrically connecting the lead of the electronic device to the pad. The pad has a predetermined width and length, and the solder deposit includes first and second end portions shaped and sized to completely cover a predetermined area corresponding to an end portion of the pad and being defined by both the entire width of the pad and a predetermined length from the end of the pad, and a connection web extending between the two end portions and integrating the two end portions into a single structure. The connection web has a longitudinal part having a width smaller than any one of the width of the pad and the width of each of the two end portions.
The solder deposit further includes first and second trapezoidal portions respectively formed at the junctions between opposite ends of the connection web and each one of the two end portions. Each of the two trapezoidal portions is symmetrically and linearly tapered at each opposite side in a manner such that the sides converge from the end portion to the connection web. The connection web has a length corresponding to the length of the lead of the electronic device mounted to the pad. The solder deposit is formed on the pad by using a screen-printing mask including a slit formed at a position corresponding to the position of the solder deposit. The slit having the same shape as the solder deposit includes first and second end opening parts individually shaped and sized to have an end coinciding with an end of the pad, each of the end opening parts also having a width equal to the width of the pad. A web opening part extends between the two end opening parts and allows the two end opening parts to communicate with each other through the web opening part. This web opening part is a longitudinal opening part having a width smaller than any one of both the width of the pad and the width of each of the two end opening parts.
The slit further includes first and second trapezoidal opening parts respectively formed at the junctions between opposite ends of the web opening part and the two end opening parts. Each of the two trapezoidal opening parts is symmetrically and linearly inclined at its opposite side edges in a way such that the side edges converge to the web opening part.